1. Field of the Invention
The present invention relates to a distributed feedback semiconductor laser (hereinafter, referred to as a "DFB laser") suitable as a light source for long distance, large capacity optical communication, and to a method for fabricating the same.
2. Description of the Related Art
Recently, DFB semiconductor lasers have been in a practical use as a light source for long distance, large capacity data transfer and for multiple channel video transfer for CATV and the like. Some of the reasons for such wide use of DFB semiconductor lasers will be described below. While usual Fabry-Perot semiconductor lasers oscillate light in a multiple longitudinal mode, DFB lasers oscillate light in a single longitudinal mode even in high speed modulation, due to a diffraction grating formed in the vicinity of an active layer. Accordingly, the noise level is low, and signals are well protected against deterioration which otherwise would be caused by dispersion during signal transfer.
Currently, refractive index-coupled DFB lasers are primarily used, in which the diffraction grating is realized by periodically changing the thickness of an optical waveguide layer located above or below the active layer. In such a refractive index-coupled DFB laser, there are two oscillation modes: at a wavelength longer than the Bragg wavelength; and at a wavelength shorter than the Bragg wavelength. The Bragg wavelength is determined by the period of the diffraction grating and the refractive index of the cavity. Either one of the oscillation modes is used by the phase at an end face of the cavity. Accordingly, stable single longitudinal mode oscillation is obtained at approximately 30%. Even though a single longitudinal mode oscillation is obtained at a low output, the oscillation mode is changed at a high output by phase fluctuation caused by an axial hole burning effect, thereby reducing the yield for obtaining stable single longitudinal mode oscillation in a wide range from a low output to a high output. In the case when emitted light is partially returned to the laser, the oscillation state changes to raise the noise level or change the single longitudinal mode oscillation into the multiple longitudinal mode oscillation. In order to establish stable single longitudinal mode oscillation under such circumstances, the refractive index-coupled DFB laser is presented in a form of a module including a built-in optical isolator for practical use. However, the low yield and the use of the optical isolator increase production cost, thus preventing wider use of DFB lasers.
In recent years, gain-coupled DFB lasers have been a focus of attention as a DFB laser having a novel structure for solving the above-described problems of refractive index-coupled DFB lasers (See, for example, Yi Luo et al., Applied Physics Letters, vol. 56, No. 17, pp. 1620-1622, Apr. 23, 1990.). In the structure of gain-coupled DFB lasers, the Bragg wavelength is basically used as the oscillation wavelength. Accordingly, stable single longitudinal mode oscillation is obtained at a high yield with no influence of the phase fluctuation at an end face of the cavity. Gain-coupled DFB lasers are not easily influenced by the phase fluctuation caused by the axial hole burning effect, either, which also contributes to a high yield of obtaining the stable single longitudinal mode oscillation mode at a high output and stable operation against the light returned to the DFB laser. In consideration of these facts, gain-coupled DFB lasers can be expected to be a light source operable in a single oscillation mode and produced by a lower cost. Gain-coupled DFB lasers are available in primarily two structures. In one structure, a diffraction grating is formed in an active layer; and in the other structure, a diffraction grating includes an absorptive layer. The latter structure is more promising for practical use in consideration of oscillation at a low threshold level and high reliability.
Briefly referring to FIG. 1, an example of a gain-coupled DFB laser 50 having the latter structure will be described. FIG. 1 is a cross sectional view of the gain-coupled DFB laser 50. As is shown in FIG. 1, the gain-coupled DFB laser 50 includes an n-GaAs substrate 31, an n-GaAlAs first cladding layer 32, a GaAs/GaAlAs SCH-MQW active layer 33, a p-GaAlAs first barrier layer 34, a p-GaAlAs second barrier layer 35, a p-GaAlAs optical waveguide layer 36, a p-GaAlAs second cladding layer 37, and an n-GaAs absorptive diffraction grating 41. The layers 31 through 37 are epitaxially grown in this order, and the n-GaAs absorptive diffraction grating 41 is buried between the p-GaAlAs second barrier layer 35 and the p-GaAlAs optical waveguide layer 36. Due to the n-GaAs absorptive diffraction grating 41, the periodical change in the absorption coefficient causes the periodical changes in the gain, and thus gain-coupled oscillation is obtained.
Referring now to FIGS. 2 and 3, a method for fabricating the gain-coupled DFB laser 50 will be described. In FIG. 2, reference numeral 51 denotes an epitaxial substrate including the layers 32 through 35 as referred to in FIG. 1. On the epitaxial substrate 51, the n-GaAs absorptive layer 40 is formed. As is shown in FIG. 2, a resist layer 52 having a periodical pattern is formed on the n-GaAs absorptive layer 40 by an EB exposure method. Then, as is shown in FIG. 3, the epitaxial substrate 51 is etched by dry etching down to a level below the bottom of the n-GaAs absorptive layer 40 at areas which are not covered with the resist layer 52, thereby forming projections. After the resist layer 52 is removed, the GaAs absorptive diffraction grating 41 is on top of the projections. Next, the layers 36 and 37 (FIG. 1) are epitaxially grown to produce the gain-coupled DFB laser 50.
It is known that the duty ratio (the ratio of the size of the light absorption area with respect to the pitch of each period of the diffraction grating; represented by a/b in FIG. 3) is preferably approximately 0.1 to 0.2 in order to obtain satisfactory characteristics of the DFB laser 50. It is very important that the diffraction grating should be shaped uniformly for the purpose of attaining such a preferable duty ratio.
The pitch of each period of the diffraction grating is required to be approximately 0.2 .mu.m to 0.4 .mu.m, and the pitch of the pattern of the resist layer 52 should be as microscopic as 0.1 .mu.m to 0.2 .mu.m. Although it is possible to use the EB exposure method to form such a microscopic pattern uniformly, the use of the EB exposure method is not practical because of the long time required for exposure and the high cost of the apparatus. Dry etching is superior to wet etching in controllability, but damages a part of the semiconductor layer and reduces the reliability of the finished DFB laser.
It is more practical to form the resist layer 52 having the periodical pattern by holographic exposure and etch the semiconductor layer by wet etching. However, in the case that the holographic exposure and wet etching are used, the pitch of the resist pattern is dispersed as is shown in FIG. 2 as well as the etching pitch. As a result, the diffraction grating is largely dispersed in shape in the wafer and even disappears in some areas. The distribution of the absorption ratio .alpha. is also largely dispersed. For these reasons, it is difficult to control the duty ratio. Due to such problems in production, it is difficult to obtain desirable characteristics at a high yield.
In order to solve these problems, a method described in, for example, Japanese Laid-Open Patent Publication No. 4-326788, has been proposed. FIG. 4 shows a cross sectional view of an absorptive diffraction grating and the vicinity thereof of a DFB laser produced in such a method. According to this method, as is shown in FIG. 4, the absorptive diffraction grating includes a plurality of projections and grooves and a quantum well absorptive layer 56 provided on the projections and the grooves. The thickness of the quantum well absorptive layer 56 changes in accordance with the projections and the grooves in order to surely change the absorption ratio periodically. In such a structure, the partial disappearance of the absorptive layer is restricted. Nonetheless, due to the diffraction grating layer formed at a surface of an optical waveguide layer 55 on an active layer 54, the dispersion of etching depth causes the dispersion of the thickness of a trough part 56V of the absorptive layer 56 and also the dispersion of the distance between the absorptive layer 56 and the active layer 54. Consequently, it is difficult to control the gain coupling coefficient of such a DFB laser.
Further, the dispersion of the thickness of the optical waveguide layer 55 caused by etching is disadvantageous in controlling the refractive index coupling coefficient. It is important to perform additional etching after the resist is removed in order to adjust the height d of the projection formed at the optical waveguide layer 55 and also in order to form a satisfactory epitaxial layer on the absorptive layer 56. However, in the structure shown in FIG. 4, it is almost impossible to perform additional etching since such additional etching would reduce or, in an extreme case, nullify the thickness of the optical waveguide layer 55. Dry etching is desirable in forming the absorptive layer 56 in a preferable shape but can possibly damage the active layer 54.
In other gain-coupled DFB lasers, the gain of the active layer is periodically changed without using the absorptive diffraction grating. A representative structure of such DFB lasers is reported in Optoelectronics Conference Digest, p. 402 (1994). In such DFB lasers, a gain diffraction grating is realized by processing the active layer. Accordingly, a larger gain coupling is obtained than in the structure having an absorptive diffraction grating. Satisfactory characteristics are obtained with no extra increase in the absorption ratio. Nonetheless, growth of a p-InP cladding layer performed after the active layer is etched, a defect or accumulation of foreign substances occurs in the vicinity of the p-n junction. Such a defect or accumulation of foreign substances reduces the light emitting efficiency when the electric current is injected or shortens the life of the DFB laser.
As has been described, in conventional DFB lasers, the shape of the diffraction grating is dispersed in even one laser. In consequence, the lasers do not have satisfactory characteristics and thus are not reliable. Further, conventional DFB lasers have low production yield and thus are poor in mass production.